1. Technical Field
The present invention relates to an electrophoretic display technique for changing visual condition by controlling electric field application to charged particles, especially titania particles that are suitable as charged particles, as well as a method for making such particles.
2. Related Art
There has been an electrophoretic solution used in an electrophoretic display device wherein two kinds of dispersed particles having different colors and different charging polarities are dispersed in a colorless dispersion medium. When such two kinds of dispersed particles having different electrification charges are dispersed in one dispersion medium, the complete prevention of cohesion between particles is difficult. However, for the purpose of improving dispersibility, surface finishing is performed using a surface finishing agent.
As such a surface finishing agent, general surface finishing agents have been used. JP-A-2001-56653 (Paragraph 0026, etc.) is an example of related art. In the related art example, a multifunctional surface finishing agent having a plurality of functional groups that react with hydroxyl groups existing on the surfaces of dispersed particles has been used, without giving any special consideration on the number of functional groups, etc. contained in the surface finishing agent. If the reaction of all functional groups contained in the surface finishing agent is completed successfully by performing surface finishing of dispersed particles using such a multifunctional surface finishing agent, the surface finishing agent works as stable agent with more number of bonding groups. Therefore, such a surface finishing agent has been considered preferable for making charged particles to be used in an electrophoretic display device.
In the case of using the above multifunctional surface finishing agent, however, a problem has frequently occurred that some of the functional groups fail to react and remain as unreacted functional groups or turn into other types of functional groups. The use of dispersed particles that are made under such circumstances in an electrophoretic display solution has caused troubles. For example, dispersed particles having unreacted groups, react with each other, or dispersed particles having unreacted hydroxyl groups and other dispersed particles having other unreacted groups react with each other in a dispersion medium. Such troubles tend to cause cohesion. In case of such cohesion, the size of particles becomes larger. As a result, displayed images become rougher or the cohered particles are fixed on the substrate.